PNNL

Abstract

Microbial reduction of structural Fe(III) in nontronite was studied in batch cultures under non-growth condition using Shewanella putrefaciens, strain CN32. The rate and extent of structural Fe(III) reduction was examined as a function of electron acceptor [Fe(III)] and bacterial concentration. Fe(II) sorptions onto nontronite and CN32 cells were independently measured and well-described by the Langmuir expression with affinity constant 2.3 and 2.25 for nontronite and cells, respectively. The Fe(II) sorption capacity of nontronite, however, decreased with increasing nontronite concentration, suggesting particulate agglomeration effect. An empirical equation for sorption capacity was derived from the sorption isotherms at different nontronite concentrations and was used to calculate the 'effective' Fe(III) concentration for bioreduction. The initial rate of microbial reduction was found to be first order with respect to the 'effective' Fe(III) concentration. A kinetic biogeochemical model was assembled that incorporated the first order rate expression with respect to the ‘effective’ Fe(III) concentration, rates and extent of Fe(II) sorption to cell and nontronite surfaces, and the empirical equation for sorption capacity. The model successfully described the experimental results of microbial reduction of nontronite with variable nontronite concentrations. The microbial reduction rate, after normalized to cell concentration, however, decreased with increasing cell concentration, indicating that cell concentration did not linearly affect the reduction as commonly assumed in literature. A nonlinear, saturation-type rate expression with respect to cell concentration was needed to model bioreduction at variable cell concentration. Our results indicated that the kinetics of microbial reduction of structual Fe(III) in nontronite can be modeled after consideration of Fe(II) production and sorption, its role in inhibiting further Fe(III) reduction, and nonlinear effect of cell concentration.